Coating composition for flexible plastic film

ABSTRACT

The present invention relates to a coating composition for a flexible plastic film, and more specifically to a coating composition for producing a flexible plastic film having excellent flexibility while exhibiting high hardness. According to the present invention, the coating composition exhibits flexibility, bending property, high hardness, scratch resistance and high transparency, and hardly has a risk of damaging the film even in repetitive, continuous bending or long-time folding state, and thereby can be usefully applied to flexible mobile devices, display devices, front face and display unit of various instrument panels, and the like.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims the benefit of priority based on KoreanPatent Application No. 10-2015-0109700 filed on Aug. 3, 2015, KoreanPatent Application No. 10-2015-0160673 filed on Nov. 16, 2015 and KoreanPatent Application No. 10-2016-0098074 filed on Aug. 1, 2016 with theKorean Intellectual Property Office, the disclosures of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION (a) Field of the Invention

The present invention relates to a coating composition for a flexibleplastic film. More specifically, the present invention relates to acoating composition for producing a flexible plastic film havingexcellent flexibility while exhibiting high hardness.

(b) Description of the Related Art

Recently, with the development of mobile devices such as smartphones andtablet PCs, thinning and slimming of substrates for display arerequired. Glass or tempered glass is commonly used as a material havingexcellent mechanical properties on windows or front boards for displaysof mobile devices. However, the glass causes the weight increase of themobile devices due to its own weight, and has a problem of breakage dueto an external impact.

Therefore, plastic resin is being studied as a substitute for glass. Theplastic resin composition is lightweight but hardly has a risk ofcracking and thus are suitable for the trend of pursuing lighter mobiledevices. In particular, in order to implement a film having highhardness and abrasion resistance properties, a composition for coating ahard coating layer made of a plastic resin onto a support substrate hasbeen proposed.

As a method of improving the surface hardness of the hard coating layer,a method of increasing the thickness of the hard coating layer can heconsidered. In order to ensure the surface hardness enough to replacethe glass, it is necessary to implement a thickness of a certain hardcoating layer. However, as the thickness of the hard coating layer isincreased, the surface hardness may be increased but the occurrence ofwrinkles and curls are increased due to curing shrinkage of the hardcoating layer, and at the same time cracking and peeling of the coatlayer are likely to occur. Therefore, the practical application of thismethod is not easy.

Korean Patent Laid-Open Publication No. 2010-0041992 discloses a plasticfilm composition using a binder resin containing an ultraviolet-curablepolyurethane acrylate-based oligomer and eliminating monomers. However,the plastic film disclosed above has insufficient strength to replace aglass panel of a display with a pencil hardness of about 3H.

Meanwhile, a display in which a part of the display device is bent orflexibly warped for aesthetic and functional reasons has recently beenattracting attention, and this tendency is noticeable particularly inmobile devices such as smartphones and tablet PCs. However, since glassis not suitable for use as a cover plate for protecting such a flexibledisplay, it needs to he replaced with a plastic resin or the like.However, for that purpose, it is not easy to produce a thin film havingsufficient flexibility while exhibiting a glass level of high hardness.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a coatingcomposition capable of producing a flexible plastic film havingexcellent flexibility while exhibiting high hardness.

In order to achieve the above object, the present provides a coatingcomposition comprising:

a 3- to 6-functional acrylate-based binder;

a 7- to 20-functional urethane acrylate-based binder;

a photoinitiator; and

inorganic fine particles having a bimodal particle size distributionincluding a first inorganic fine particle group having d₅₀ of 20 to 35nm and a second inorganic fine particle group having d₅₀ of 40 to 130nm.

The coating composition according to the present invention exhibitsflexibility, bending property, high hardness, scratch resistance andhigh transparency, and hardly has of damaging the film even inrepetitive, continuous bending or long-time folding state. Therefore,the coating composition can be usefully applied to bendable, flexible,rollable or foldable mobile devices, display devices, front face anddisplay unit of various instrument panels, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the particle size distribution of the firstand second inorganic fine particle groups according to one embodiment ofthe present invention.

FIG. 2 is a graph showing the particle size distribution of theinorganic fine particles according to one embodiment of the presentinvention.

FIG. 3 is a graph showing the particle size distribution of theinorganic fine particles according to the comparative example of thepresent invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The coating composition of the present invention comprises: a 3- to6-functional acrylate-based binder; a 7- to 20-functional urethaneacrylate-based binder; a photoinitiator; and inorganic fine particleshaving a bimodal particle size distribution including a first inorganicfine particle group having d₅₀ of 20 to 35 nm and a second inorganicfine particle group having d₅₀ of 40 to 130 nm.

In the present invention, the terms such as first, second, etc. are usedto describe various components, and the terms are used only for thepurpose of distinguishing one component from another component.

Moreover, the terminology used herein is for the purpose of describingexemplary embodiments only and is not intended to limit the presentinvention. The singular expression includes plural expressions unlessthe context clearly indicates otherwise. It will be understood that theterms such as “comprise,” “include,” “have,” etc. as used herein specifythe presence of stated features, integers, steps, components, orcombinations thereof but do not preclude the presence or addition of oneor more other features, integers, steps, components, and/or combinationsthereof.

As the present invention allows for various changes and numerousembodiments, particular embodiments will be illustrated and described indetail below. However, this is not intended to limit the presentinvention to particular modes of practice, and it is to be appreciatedthat all changes, equivalents, and substitutes that do not depart fromthe spirit and technical scope of the present invention are encompassedin the present invention.

The coating composition of the present invention will be described inmore detail below.

The coating composition according to an embodiment of the presentinvention comprises: a 3- to 6-functional acrylate-based hinder; a 7- to20-functional urethane acrylate-based binder; a photoinitiator; andinorganic fine, particles having a bimodal particle size distributionincluding a first inorganic fine particle group having d₅₀ of 20 to 35nm and a second inorganic fine particle group having d₅₀ of 40 to 130nm.

First, the coating composition of the present invention includes a 3- to6-functional acrylate-based binder.

As used herein, the acrylate-based means not only acrylate but alsomethacrylate, or derivatives in which a substituents are introduced intoacrylate or methacrylate.

The 3- to 6-functional acrylate-based binder is cross-linked with the 7-to 20-functional urethane acrylate-based hinder to form a copolymer, andcan impart high hardness to the coating layer formed after curing.

More specifically, the 3- to 6-functional acrylate-based binder mayinclude trimethylolpropane triacrylate (TMPTA), trimethylolpropaneethoxytriacrylate (TMPEOTA), glycerin propoxylated triacrylate (GPTA),pentaerythritol tetraacrylate (PETA), dipentaerythritol hexaacrylate(DPHA), and the like, but the present invention is not limited thereto.The above-mentioned 3- to 6-functional acrylate-based binder may be usedalone or in combination of different types.

According to one embodiment of the present invention, the 3- to6-functional acrylate-based binder has a weight average molecular weight(Mw) ranging from about 200 to about 2,000 g/mol, or from about 200 toabout 1,000 g/mol, or from about 200 to about 500 g/mol.

According to one embodiment of the present invention, the 3- to6-functional acrylate-based binder has an acrylate equivalent weightranging from about 50 to about 300 g/mol, or about 50 to about 200g/mol, or about 50 to about 150 g/mol.

When the weight average molecular weight and the acrylate equivalentweight of the 3- to 6-functional acrylate-based binder are respectivelywithin the above-mentioned ranges, a coating layer having more optimizedphysical properties can be formed.

The 3- to 6-functional acrylate-based binder can be contained in anamount of about 10 to about 50 parts by weight, or about 10 to about 40parts by weight, or about 10 to about 30 parts by weight, based on 100parts by weight of the entire coating composition. When content of the3- to 6-functional acrylate-based binder is too small, the hardness ofthe coating layer can he lowered, and if the content is too high, thebending property or the flexibility may be deteriorated. From thisviewpoint, it is desirable to include the binder in a ratio of the partsby weight described above.

The coating composition of the present invention includes a 7- to20-functional urethane acrylate-based binder.

The 7- to 20-functional urethane acrylate-based binder is cross-linkedwith the 3- to 6-functional acrylate-based binder to form a copolymer,and may impart high hardness, flexibility and impact resistance to thecoating layer formed after curing. The 7- to 20-functional urethaneacrylate-based binder may he used alone or in combination of differenttypes.

According to one embodiment of the present invention, the coatingcomposition may contain the 3- to 6-functional acrylate-based binder andthe 7- to 20-functional urethane acrylate-based binder in a ratio ofabout 1:9 to about 5:5, preferably 1:9 to about 4:6, more preferablyfrom about 1:9 to about 3.5:6.5. By including the 3- to 6-functionalacrylate-based binder and the 7- to 20-functional urethaneacrylate-based binder in the weight ratio described above, it canexhibit sufficient flexibility and at the same time achieve highhardness and good physical properties.

According to one embodiment of the present invention, the 7- to20-functional urethane acrylate-based hinder has a weight averagemolecular weight ranging from about 2,000 to about 8,000 g/mol, or fromabout 3,000 to about 6,000 g/mol, or from about 3,000 to about 5,000g/mol, which may be preferable for the optimization of physicalproperties of the coating layer.

According to an embodiment of the present invention, the 7- to20-functional urethane acrylate-based binder may have an acrylateequivalent weight ranging from about 200 to about 1,500 g/mol, or fromabout 200 to about 1,000 g/mol, or from about 300 to about 600 g/mol, orfrom about 300 to about 500 g/mol. If the acrylate equivalent weight ofthe 7- to 20-functional urethane acrylate-based binder is too high, thehardness of the coating layer may be insufficient, and if the equivalentweight is low, the hardness may be improved but the flexibility may bedeteriorated. From the viewpoint of the balance between high hardnessand flexibility as described above, the above-mentioned range ofequivalent weight is preferable, and about 300 to about 500 g/mol may bemost preferable.

When the weight average molecular weight and the acrylate equivalentweight of the 7- to 20-functional urethane acrylate-based binder arerespectively within the ranges described above, coating layers havingmore optimized properties can be formed.

As the 7- to 20-functional urethane acrylate-based binder contains inthe molecule at least 7 acrylate groups capable of performing acrosslinking polymerization by ultraviolet rays, it is advantageous forachieving high hardness of the coating layer. However, the higher thecross-linking density, it allows the curls to be easily generated andthe adhesive force with the substrate is lowered, and so it is notsuitable for forming a flexible film.

Meanwhile, the 7- to 20-functional urethane acrylate-based bindercontained in the coating composition of the present invention includesat least 7 polyfunctional acrylate groups and at the same time has aurethane bond in the molecule, and thus is excellent in elasticity andflexibility. Accordingly, when it is cross-linked with a 3- to6-functional acrylate-based hinder at an appropriate weight ratio toform a copolymer, it serves to impart sufficient flexibility togetherwith high hardness to the coating layer. The 7- to 20-functionalurethane acrylate-based binder may contain 2 to 20 urethane bonds in onemolecule.

As such, the coating composition of the present invention includes the3- to 6-functional acrylate-based binder and the 7 to 20-functionalurethane acrylate-based binder, thereby imparting high hardness andflexibility to the coating layer and the flexible plastic film formed7therefrom. In particular, it has high durability against bending,rolling or folding, and thus it is possible to secure excellentflexibility, which hardly has a risk of damaging the film even whenrepeatedly warped or folded for a long time.

The 7 to 20-functional urethane acrylate-based binder can be containedin an amount of about 40 to about 70 parts by weight, or about 40 toabout 75 parts by weight, or about 40 to about 65 parts by weight, basedon 100 parts by weight of the entire coating composition. When contentof the 7- to 20-functional urethane acrylate-based hinder is too small,the coating layer does not exhibit sufficient flexibility and thus thebending property may be deteriorated, and if the content is too high,the surface hardness of the coating layer may be lowered. From thisviewpoint, it is desirable to include the binder in a ratio of the pailsby weight described above.

The coating composition of the present invention includes aphotoinitiator.

Examples of the photoinitiator may include 1-hydroxy-cyclohexyl-phenylketone, 2-hydroxy-2-methyl-1-phenyl-1-propanone,2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone,methylbenzoylformate, α,α-dimethoxy-α-phenylacetophenone,2-benzoyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone,2-methyl-1-[4-(methylthio) phenyl]-2-(4-morpholinyl)-1-propanonediphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide, orbis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, and the like, but arenot limited thereto. In addition, examples of commercially availableproducts include Irgacure 184, Irgacure 500, Irgacure 651, Irgacure 369,Irgacure 907, Darocur 1173, Darocur MBF, Irgacure 819, Darocur TPO,Irgacure 907, Esacure KIP 100F, and the like. These photoinitiators canbe used alone or in combination of two or more.

The content of the photoinitiator is not particularly limited, but inorder to achieve effective photopolymerization without impairing thephysical properties of the entire coating composition, thephotoinitiator may be contained in an amount of about 0.1 to about 5parts by weight based on 100 parts by weight of the coating composition.

The coating composition of the present invention comprises inorganicfine particles having a bimodal particle size distribution including afirst inorganic fine particle group having d₅₀ of 20 to 35 nm and asecond inorganic fine particle group having d₅₀ of 40 to 130 nm. Asdescribed above, the coating composition of the present invention usesthe inorganic fine particles exhibiting a bimodal particle sizedistribution including the first and second inorganic fine particlegroups each having a specific range of d₅₀, thereby improving thehardness and flexibility of the coating layer simultaneously whilemaintaining the flexible property.

In the specification of the present invention, when a cumulativeparticle size distribution corresponding to particle sizes was measuredusing a laser light diffraction method (measurement method: sizedistribution by number is determined by using dynamic laser scattering,a solvent in which inorganic fine particles are dispersed, refractiveindex, viscosity, and dielectric constant of the inorganic fineparticles, the equipment name: Malvern Zetasizer Nano-ZS 90), theparticle size at the 10% cumulative is set to d₁₀, the particle size atthe 50% cumulative to d₅₀, and the particle size at the 90% cumulativeto d₉₀. The particle size distribution by the laser light diffractionmethod can show substantially the same distribution as that measuredwith SEM or TEM by diluting a dispersion liquid in which inorganic fineparticles are dispersed in a solvent, or measured by analyzing the crosssection of the coating layer containing the inorganic fine particles bySEM or TEM.

The first inorganic fine particle group having the small particle sizerange contributes to the improvement of the hardness, and the secondinorganic fine particle group having the larger particle size rangecontributes to the improvement of bending property and flexibility. Inthis way, as other inorganic fine particle groups having differentparticle size ranges are mixed and used in addition to the cross-linkedcopolymer described above, it is possible to provide a coating layer inwhich the physical properties of hardness and flexibility are improvedsimultaneously. In addition, the hardness and flexibility may differdepending on the particle size distribution of the first and secondinorganic fine particle groups, and when the first and second inorganicfine particle groups satisfying the predetermined particle sizedistribution are used, the physical property balance between thehardness and the flexibility of the coating layer may be optimized. Thepresent invention has been completed on the basis of these facts. As thefirst and second inorganic fine particle groups, for example, silicafine particles, aluminum oxide particles, titanium oxide particles, zincoxide particles or the like may be each independently used.

According to one embodiment of the present invention, the d₅₀ of thefirst inorganic fine particle group may be 20 nm or more, or about 21 nmor more, 35 nm or less, 30 nm or less, or 25 nm or less, and the d₅₀ ofthe second inorganic fine particle group may be about 40 nm or more, orabout 42 nm or more, or about 45 nm or more, and 130 nm or less, or 125nm or less, or 120 nm or less.

According to one embodiment of the present invention, the firstinorganic fine particle group may have d₁₀ of 10 to 19 nm, d₅₀ of 20 to35 nm, and a d₉₀ of 25 to 40 nm. Further, the second inorganic fineparticle group may have d₁₀ of 25 to 110 nm, d₅₀ of 40 to 130 nm, andd₉₀ of 60 to 150 nm.

According to one embodiment of the present invention, the content of thefirst inorganic fine particle group may be about 5 parts by weight ormore, or about 10 parts by weight or more, or about 15 parts by weightor more, based on 100 parts by weight of the coating layer, in order tocontribute to an improvement in high hardness. The content of the firstinorganic fine particle group may be about 50 parts by weight or less,or about 45 parts by weight or less, or about 40 parts by weight orless, or about 35 parts by weight or less, in order to satisfy theflexibility. By including the first inorganic fine particle group withinthe weight range described above, it is possible to form a flexibleplastic film having excellent physical properties simultaneouslysatisfying both high hardness and flexibility.

Further, according to one embodiment of the present invention, thecontent of the second inorganic fine particle group may be about 5 partsby weight or more, or about 10 parts by weight or more, or 15 parts byweight or more, based on 100 parts by weight of the entire coatinglayer, in order to contribute to an improvement in high hardness, andmay be about 50 parts by weight or less, or about 45 parts by weight orless, or about 40 parts by weight or less, or about 35 parts by weightor less in order to satisfy flexibility. By including the secondinorganic fine particle group within the weight range described above,it is possible to form a flexible plastic film having excellent physicalproperties simultaneously satisfying both high hardness and flexibility.

According to one embodiment of the present invention, the total contentof the inorganic fine particles including the first and second inorganicfine particle groups may be about 25 parts by weight or more, or about30 parts by weight or more, or about 35 parts by weight or more, basedon 100 parts by weight of the entire coating layer, in order tocontribute to an improvement in high hardness, and may be about 50 partsby weight or less, or about 45 parts by weight or less, or about 40parts by weight or less in order to satisfy flexibility.

According to one embodiment of the present invention, the first andsecond inorganic fine particle groups may be the same or different andeach independently surface-modified with any one or more silane couplingagents selected from the group consisting of (meth)acrylsilane,methacroxysilane, vinylsilane, epoxysilane, and mercaptosilane.

Since the first and second inorganic fine particle groupssurface-modified with the silane coupling agent as described above canreact with the acrylate group of the binder, the adhesion to thesubstrate is high, they can be uniformly dispersed in the coating layer,and the hardness can be improved without deteriorating the flexibilityof the coating layer, which are therefore more advantageous.

According to one embodiment of the present invention, the first andsecond inorganic fine particle groups may be present in the weight ratioof about 9:1 to about 3:7, or about 8:2 to about 4:6, or about 7:3 toabout 5:5. By including the first and second inorganic fine particlegroups within the weight ratio range described above, it is possible toform a flexible plastic film having excellent physical properties withenhanced high hardness and flexibility.

The coating composition may optionally further include an organicsolvent for proper fluidity and coating property.

Examples of the organic solvent include alcohol based solvents such asmethanol, ethanol, isopropyl alcohol and butanol; alkoxy alcohol basedsolvents such as 2-methoxyethanol, 2-ethoxyethanol and1-methoxy-2-propanol; ketone based solvents such as acetone, methylethyl ketone, methyl isobutyl ketone, methyl propyl ketone andcyclohexanone; ether based solvent such as propylene glycol monopropylether, propylene glycol monomethyl ether, ethylene glycol monoethylether, ethylene glycol monopropyl ether, ethylene glycol monobutylether, diethylene glycol monomethyl ether, diethyl glycol monoethylether, diethyl glycol monopropyl ether, diethyl glycol monobutyl etherand diethylene glycol-2-ethylhexyl ether; aromatic solvent such asbenzene, toluene and xylene, and the like. These may be used alone or incombination.

The content of the organic solvent is not particularly limited since itcan be variously adjusted within a range that does not deteriorate thephysical properties of the coating composition, but it can included sothat the weight ratio of the solid content: organic solvent is about30:70 to about 99:1. based on the solid content of the componentscontained in the coating composition. When the organic solvent is withinthe above range, it can have appropriate fluidity and coating property.

Meanwhile, the coating composition of the present invention may furtherinclude additives commonly used in the art, such as a surfactant, a UVabsorber, a UV stabilizer, an anti-yellowing agent, a leveling agent, anantifouling agent, a dye for improving the color value, etc., inaddition to the above-mentioned binder, inorganic fine particles,photoinitiator and organic solvent. Further, the content thereof is notparticularly limited as it can be variously adjusted within the rangethat does not deteriorate the physical properties of the coatingcomposition of the present invention. However, they may be contained inan amount of about 0.01 to 10 parts by weight based on about 100 partsby weight of the coating composition.

According to an embodiment of the present invention, for example, thecoating composition may contain a surfactant as an additive, and thesurfactant may be a mono- or bi-functional fluorine-based acrylate, afluorine-based surfactant, or a silicon-based surfactant. In this case,the surfactant may be contained in a form of being dispersed orcrosslinked in the crosslinked copolymer.

Further, the additive may include a UV absorber, a UV stabilizer, andexamples of the UV absorber include a benzophenone-based compound, abenzotriazole-based compound, a triazine-based compound and the like.Examples of the UV stabilizer include tetramethyl piperidine or thelike.

According to one embodiment of the present invention, the viscosity ofthe coating composition is not particularly limited as long as it has asuitable fluidity and coating property, and for example, it may have aviscosity of about 1,200 cps or less at a temperature of 25° C.

A coating composition comprising a 3- to 6-functional acrylate-basedbinder, a 7- to 20-functional urethane acrylate-based binder, inorganicfine particles having a bimodal particle size distribution including afirst inorganic fine particle group having d₅₀ of 20 to 35 nm and asecond inorganic fine particle group having d₅₀ of 40 to 130 nm,additives, and an organic solvent can be coated onto the substrate andthen photo-cured to form a coating layer.

The plastic film including a coating layer formed by using the coatingcomposition of the present invention as described above exhibitsflexibility, bending property, high hardness, scratch resistance andhigh transparency, and the damage of the film is small even inrepetitive, continuous bending or long-time folding state. Therefore,the plastic film can be usefully applied to bendable, flexible, rollableor foldable mobile devices, display devices, front face and display unitof various instrument panels, and the like.

In the present invention, “flexible” means a state having flexibility tosuch an extent that cracks of 3 mm or more in length do not occur whenwound on a cylindrical mandrel with a diameter of 4 mm. Therefore, theflexible plastic film including a coating layer formed by using thecoating composition of the present invention can be applied to a coverfilm of a bendable, flexible, rollable, or foldable display or the like.

The support substrate on which the coating layer is formed can be usedwithout particularly limiting the method and material for producing thesupport substrate such as a stretched film or a non-stretched film aslong as it is an optical transparent plastic resin having an elasticmodulus of about 4 GPa or more as measured according to ASTM D882 and athickness in the range of 2.0 to 200 μm so that it secures flexibilityand hardness.

Among the conditions of the support substrate, the elastic modulus maybe about 4 GPa or more, or about 5 GPa or more, or about 5.5 GPa ormore, or about 6 GPa or more, and the upper limit value may be about 9GPa or less, or about 8 GPa or less, or about 7 GPa or less. If theelastic modulus is less than 4 GPa, sufficient hardness cannot beachieved, and if the elastic modulus exceeds 9 GPa which is too high, itmay be difficult to form a film having flexibility.

The thickness of the support substrate may be about 20 μm or more, orabout 25 μm or more, or about 30 μm or more, and the upper limit valuethereof may be about 200 μm or less, or about 150 μm or less, or about100 or less. If the thickness of the support substrate is less than 20μm, there is a possibility that breakage or curling occurs in theprocess of forming the coating layer, and it may be difficult to achievehigh hardness. On the other hand, if the thickness exceeds 200 μm, theflexibility deteriorates and it may be difficult to form a flexiblefilm.

From the viewpoint of ensuring the processibility for the flexible filmand achieving the balance of physical properties between the highhardness and the flexibility as described above, a support substratehaving an elastic modulus of 4 GPa or more and 9 GPa or less and athickness in the range of 20 to 200 μm can be used.

More specifically, according to one embodiment of the present invention,the support substrate satisfies the above-described elastic modulus andthickness range, and for example, it may be a film including polyimide(PI), polyimideamide, polyetherimide (PEI), polyethyleneterephthalate(PET), polyethylenenaphthalate (PEN), polyetheretherketone (PEEK),cyclic olefin polymer (COP), polyacrylate (PAC), polymethylmethacrylate(PMMA), triacetylcellulose (TAC), and the like. The above supportsubstrate may have a single layer or a multilayer structure including,if necessary, two or more substrates composed of the same as ordifferent substances, and is not particularly limited.

Alternatively, according to one embodiment of the present invention, thesupport substrate may be a substrate comprising polyimide (PI).

Further, according to one embodiment of the present invention, thethickness ratio between the support substrate and the coating layer onthe both surfaces may be about 1:0.05 to about 1:1, or about 1:0.1 toabout 1:0.8. When the thickness ratio between the support substrate andthe coating layer is within the above range, a flexible plastic filmexhibiting high hardness and flexibility can be formed more easily.

According to one embodiment of the present invention, the flexibleplastic film may include a coating layer formed on at least one surfaceor both surfaces of the support substrate using the coating compositionof the present invention.

The coating layer comprises a crosslinked copolymer of a 3- to6-functional acrylate-based binder and a 7- to 20-functional urethaneacrylate-based binder; and inorganic fine particles having a bimodalparticle size distribution including a first inorganic fine particlegroup having d₅₀ of 20 to 35 nm and a second inorganic fine particlegroup having d₅₀ of 40 to 130 nm.

The coating composition may be each sequentially coated onto the frontand rear surfaces of the support substrate, or simultaneously coatedonto both surfaces of the support substrate.

According to one embodiment of the present invention, the coatingcomposition comprising the above-described components can be coated ontoboth surfaces of the support substrate and then photo-cured to form acoating layer, thereby obtaining the flexible plastic film of thepresent invention. Here, the method of coating the coating compositionis not particularly limited as long as it can be used in the technicalfield to which the present technology belongs, and for example, a barcoating method, a knife coating method, a roll coating method, a bladecoating method, a die coating method, a micro gravure coating method, acomma coating method, a slot die coating method, a lip coating method, asolution casting method, or the like can be used.

The coating layer may have a thickness of about 3 μm or more, forexample about 3 to about 20 μm, or about 3 to about 15 μm or about 3 toabout 10 μm after being completely cured. According to the presentinvention, it is possible to provide a flexible plastic film having ahigh hardness when a coating layer having such a thickness is included.

According to one embodiment of the present invention, the flexibleplastic film may further include at least one selected from a layer, amembrane, a film or the like such as a plastic resin film, a cohesivefilm, a release film, a conductive film, a conductive layer, a liquidcrystal layer, a coating layer, a cured resin layer, a non-conductivefilm, a metal mesh layer or a patterned metal layer on the top surfaceof at least one coating layer or between the substrate film and thecoating layer. For example, an antistatic layer having conductivity isfirst formed on a support substrate, and then a coating layer is formedthereon to provide an anti-static function, or a low refractive indexlayer is introduced on the coating layer to implement a low reflectionfunction.

Further, the layer, membrane, film or the like may be in any form of asingle layer, a double layer, or a laminate type. The layer, membrane,film or the like may be formed by laminating a freestanding film with anadhesive, a cohesive film, or the like, or may be laminated on thecoating layer by a method such as coating, vapor deposition, sputtering,or the like, but the present invention is not limited thereto.

The flexible plastic film according to one embodiment of the presentinvention can be produced, for example, by the following method.

The flexible plastic film can be formed by a two-step process of firstcoating and first photo-curing a first coating composition on onesurface of a support substrate, and then second coating and secondphoto-curing the coating composition on the other surface, that is theback side of the support substrate. At this time, the first and secondcoating compositions are the same as the above-described coatingcomposition, and each of them distinguishes the composition coated ontoonly one surface and the back surface. In the case of forming thecoating layer by this method, in the second photo-curing step,ultraviolet ray irradiation is performed not on the surface coated withthe first coating composition but on the opposite surface, and therebycurls which may be generated due to curing shrinkage in the firstphoto-curing step can be cancelled out in opposite directions to obtaina flat flexible plastic film. Therefore, no additional flatteningprocess is required.

However, the present invention is not limited thereto, and the curlbalance can also be adjusted by forming the coating compositionsimultaneously on both surfaces of the support substrate and then curingthe coating composition.

The flexible plastic film prepared by using the coating composition ofthe present invention exhibits excellent flexibility, bending property,high hardness, scratch resistance, high transparency, high durabilityand stability against bending, rolling or folding and thus can be usedas a cover film of a next generation display having bendable, flexible,rollable or foldable properties, and the like.

For example, the flexible plastic film prepared by using the coatingcomposition of the present invention exhibits flexibility to such anextent that cracks do not occur when wound on a cylindrical mandrel witha diameter of 4mm or 3 mm.

Further, the flexible plastic film prepared by using the coatingcomposition of the present invention may have a pencil hardness of 6H ormore, or 7H or more, under a load of 750 g.

In addition, in the flexible plastic film prepared by using the coatingcomposition of the present invention, when placing the film at aninterval of 4 mm in the middle of the film, allowing the film to standat room temperature while both sides of the film being folded at 90degrees toward the bottom surface, and spreading the film on a flatbottom surface, the film may exhibit a bending stability to such anextent that a height lifted from the bottom surface is about 0.5 mm orless.

Further, in the flexible plastic film prepared by using the coatingcomposition of the present invention, when placing the film at aninterval of 4 mm in the middle of the film and repeating 100,000 timesthe processes of folding and spreading both sides of the film at 90degrees toward the bottom surface, the film may exhibit a bendingdurability to such an extent that a crack of 1 cm or more or 3 mm ormore does not occur.

Further, when steel wool #0000 is attached to a tip having a contactarea of 2 cm×2 cm with respect to a plastic film in a friction testerand then the surface of the plastic film is reciprocated 400 times undera load of 500 g, 2 or less scratches can occur.

In addition, the flexible plastic film prepared by using the coatingcomposition of the present invention may have a light transmittance of88.0% or more, or 90.0% or more, and a haze of 1.5% or less, 1.0% orless, or 0.5% or less.

The flexible plastic film prepared by using the coating composition ofthe present invention can be utilized in various fields. For example,the flexible plastic film of the present invention can be used as aflat-shaped as well as a curved, bendable, flexible, rollable orfoldable-shaped mobile communication terminal, a touch panel of asmartphone or a tablet PC, and cover substrate or element substrate ofvarious displays.

Hereinafter, the operation and effect of the invention will be describedin more detail by way of concrete examples. However, these examples aremerely presented for illustrative purposes only, and the scope of theinvention is not determined thereby.

EXAMPLES Example 1

30 g of trimethylolpropane triacrylate (TMPTA) (manufactured by Cytec,Mw=296 g/mol, acrylate group equivalent weight=99 g/mol) as atrifunctional acrylate-based binder, 40 g of MU9800 (manufactured byMiwon, Mw=3500 g/mol, acrylate group equivalent weight=389 g/mol) as a9-functional urethane acrylate-based binder, 30 g of MU9020(manufactured by Miwon, Mw=4500 g/mol, acrylate group equivalentweight=450 g/mol) as a 10-functional urethane acrylate-based binder, 1 gof Irgacure 184 (manufactured by Ciba) as a photoinitiator, and 15 g ofmethyl ethyl ketone (MEK) were mixed to prepare an acrylate solution.

60 g of a solution in which a silica particle S (d₁₀=17 nm, d₅₀=22 nm,d₉₀=28 nm, surface-modified with methacrylate silane coupling agent) wasdispersed in n-BA 3 (normal butyl acetate) in an amount of 50% by weight(hereinafter, referred to as S1 dispersion solution), and 50 g of asolution in which a silica particle S2 (d₁₀=29 nm, d₅₀=51 nm, d₉₀=74 nm,surface-modified with an acrylate silane coupling agent) was dispersedin MEK in an amount of 30% by weight (hereinafter, referred to as S2dispersion solution) were mixed with the resulting acrylate solution toprepare a coating composition.

The coating composition was coated onto both surfaces of a polyimidesubstrate (size: 20 cm×30 cm, thickness: 35 μm) having an elasticmodulus value of 6.0 GPa as measured according to ASTM D882 by a barcoating method, and photo-cured with a metal halide lamp having awavelength of 290 to 320 nm to form a coating layer.

After the curing was completed, the thickness of the coating layerformed on both surfaces was 6 μm, respectively.

Example 2

The coating layer was formed in the same manner as in Example 1, exceptthat the S2 dispersion solution was used in an amount of 83.3 g of andno additional methyl ethyl ketone solvent was contained.

Example 3

In Example 1, 25 g of methyl ethyl ketone was used, and 37.5 g of asolution in which a silica particle S3 (d₁₀=108 nm, d₅₀=119 nm, d₉₀=131nm, surface-modified with an acrylate silane coupling agent) wasdispersed in MEK in an amount of 40% by weight (hereinafter, referred toas S3 dispersion solution) was mixed instead of 50 g of the S2dispersion solution to prepare a coating composition.

The subsequent steps were carried out in the same manner as in Example 1to form a coating layer.

Example 4

20 g of TMPTA (manufactured by Cytec, Mw=296 g/mol, acrylate groupequivalent weight=99 g/mol), 40 g of MU9800 (manufactured by Miwon,Mw=3500 g/mol, acrylate group equivalent weight=389 g/mol), 40 g ofMU9020 (manufactured by Miwon, Mw=4500 g/mol, acrylate group equivalentweight=450 g/mol), 1 g of Irgacure 184 (manufactured by Ciba) as aphotoinitiator and 12 g of methylethyl ketone (MEK) were mixed toprepare an acrylate solution.

60 g of the S1 dispersion solution and 75 g of the S3 dispersionsolution were mixed with the resulting acrylate solution to prepare acoating composition.

The subsequent steps were carried out in the same manner as in Example 1to form a coating layer.

Example 5

30 g of TMPTA (manufactured by Cytec, Mw=296 g/mol, acrylate groupequivalent weight=99 g/mol), 30 g of MU9020 (manufactured by Miwon,Mw=4500 g/mol, acrylate group equivalent weight=450 g/mol), 40 g of SC2152 (manufactured by Miwon, Mw=20,000 g/mol, acrylate group equivalentweight=1,333 g/mol) as a 15-functional urethane acrylate-based binder, 1g of Irgacure 184 (manufactured by Ciba) as a photoinitiator, and 42 gof methyl ethyl ketone (MEK) were mixed to prepare an acrylate solution.

60 g of the S1 dispersion solution and 83.3 g of the S2 dispersionsolution were mixed with the resulting acrylate solution to prepare acoating composition.

The subsequent steps were carried out in the same manner as in Example 1to form a coating layer.

Comparative Example 1

The coating layer was formed in the same manner as in Example 1, exceptthat methyl ethyl ketone was used in an amount of 55 g and a silicaparticle was not contained in the coating composition.

Comparative Example 2

The coating layer was formed in the same manner as in Example 1, exceptthat methyl ethyl ketone was used in an amount of 12 g and only asolution in which a silica particle S4 (d₁₀=12 nm, d₅₀=17 nm, d₉₀=21 nm,surface-modified with an acrylate silane coupling agent) was dispersedin MEK (hereinafter, referred to as S4 dispersion solution) was used inan amount of 112.5 g.

Comparative Example 3

The coating layer was formed in the same manner as in Example 1, exceptthat methyl ethyl ketone was used in an amount of 35 g and only a S1dispersion solution was contained in an amount of 110 g.

Comparative Example 4

The coating layer was formed in the same manner as in Example 1, exceptthat a S3 dispersion solution was used in an amount of 125 g, a S4dispersion solution in an amount of 25 g, and an additional methyl ethylketone solvent was not contained.

Comparative Example 5

Except for using 70 g of PS610 (manufactured by Miwon, Mw=5,400 g/mol,acrylate group equivalent weight=900 g/mol) as a 6-functional polyesteracrylate-based binder instead of MU9800 and MU9020 in ComparativeExample 3, the other steps were carried out in the same manner as inComparative Example 3 to form a coating layer.

The main components of the coating compositions of Examples 1 to 5 andComparative Examples 1 to 5 are summarized Tables 1 and 2 below,respectively.

The particle size distribution graph of the entire inorganic fineparticles contained in the coating compositions of Examples 1 to 4 isshown in FIG. 2, and the particle size distribution graph of the entireinorganic fine particles contained in the coating compositions ofComparative Examples 2 to 4 is shown in FIG. 3.

In Examples and Comparative Examples, the particle size distribution(d₁₀, d₅₀, d₉₀) of the inorganic fine particles was measured in thestate of a dispersed solution using Malvern Zetasizer Nano-ZS 90 todetermine a size distribution by number.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Acrylate basedTMPTA 30 30 30 20 30 binder MU9800 40 40 40 40 — (unit: g) MU9020 30 3030 40 30 SC2152 — — — — 40 Inorganic fine S1 30 30 30 30 30 particles*S2 15 25 — — 25 (unit: g) S3 — — 15 30 — S4 — — — — —

TABLE 2 Comparative Comparative Comparative Comparative ComparativeExample 1 Example 2 Example 3 Example 4 Example 5 Acrylate TMPTA 30 3030 30 30 based MU9800 40 40 40 40 — binder MU9020 30 30 30 30 — (unit:g) PS610 — — — — 70 Inorganic S1 — — 55 — 55 fine S2 — — — — —particles* S3 — — — 50 — (unit: g) S4 — 45 — 10 —

*In Table 1 and 2, the content of the inorganic fine particles isrepresented by the net weight of only the inorganic fine particles (S1to S4) excluding the solvent according to the weight percentage of theinorganic fine particles dispersed in the solvent.

Experimental Example

<Measurement Method>

1) Pencil Hardness

The maximum hardness without scratches was confirmed after moving thepencil back and forth three times at an angle of 45 degrees under a loadof 750 g using a pencil hardness tester in accordance with standard JISK5400-5-4.

2) Transmittance and Haze

The transmittance and haze were measured using a spectrophotometer(apparatus name: COH-400).

3) Bending Test

Each film was interposed and wound between cylindrical mandrels ofvarious diameters and then the minimum diameter at which no cracksoccurred was measured.

4) Bending Durability Test

Each of the films of Examples and Comparative Examples was cut, butlaser cutting was performed into a size of 80×140 mm so as to minimizefine cracks at the edge portions. The laser cut film was placed on themeasuring equipment and set so that the interval between the foldedportions was 4 mm. Then, processes of folding and spreading both sidesof the films at 90 degrees toward the bottom surface at room temperaturewere repeated 10,000 times by continuous operations (the speed at whichthe film was folded was once every 1.5 seconds)

After repeating 10,000 times, the film was peeled off, and it wasobserved whether or not cracks of 3 mm or more in length occurred (OK,NG). When cracks did not occur, the film was again bended 10,000 timesand whether or not cracks occurred was repeatedly observed, therebymeasuring the maximum number of repetitions that cracks do not occur.When cracks did not occur up to 100,000 times of repetitions, thebending durability was judged to be excellent.

5) Bending Stability Test

Similarly to the bending durability test, each of the films of Examplesand Comparative Examples was cut, but laser cutting was performed into asize of 80×140 mm so as to minimize fine cracks at the edge portions.

The laser cut film was placed on the fixing device and set so that theinterval between the folded portions was 4 mm. Then, the film wasallowed to stand at room temperature for 24 hours while both sides ofthe film being folded at 90 degrees to the bottom of the film. The filmwas then peeled off and the film was turned over so that the foldedportion went downward. The

-shaped SUS structure was placed thereon and the film was fixed. The 3Dimage of the film shape was measured with a noncontact-type surfaceroughness measuring instrument (PLUTO 681, Dukin Co., Ltd., use of 605nm laser, resolution 0.1 μm), and the maximum height of the height Zlifted from the bottom was measured as the bending stability property.

6) Recovery Test

In order to measure the recovery of the films, the films in which thebending durability and stability properties were measured were allowedto stand at room temperature for 1 hour, and the maximum value of thelifted height Z was again measured according to 5) bending stabilitytest method. The change in the appearance of the folded portion wasvisually observed.

If Z is 0.1 mm or less and appearance change such as traces of foldedparts is insufficient, it is shown as OK, and if Z exceeds 0.1 mm or alarge number of traces remain on the folded portions, it is shown as NO.

The measurement results of the physical properties are shown in Tables 3and 4 below.

TABLE 3 Exam- ple 1 Example 2 Example 3 Example 4 Example 5 Pencil 7H 8H7H 8H 6H hardness Haze  0.4%  0.3%  0.4%  0.4%  0.4% Transmittance 91.9%91.8% 92.0% 91.9% 91.7% Bending test   4 mm   4 mm   4 mm   4 mm   3 mmBending 100,000 100,000 100,000 100,000 100,000 durability times timestimes times times OK OK OK OK OK Bending 0.2 mm 0.2 mm 0.2 mm 0.2 mm 0.1mm stability Recovery OK OK OK OK OK

TABLE 4 Comparative Comparative Comparative Comparative ComparativeExample 1 Example 2 Example 3 Example 4 Example 5 Pencil 4H 5H 8H 5H 8Hhardness Haze  0.2%  0.3%  0.4%  0.4%  0.3% Transmittance 92.1% 91.9%91.8% 91.8% 92.0% Bending test   3 mm   4 mm   5 mm   4 mm   8 mmBending 100,000 100,000 NG 100,000 NG durability times times (10,000times) times (10,000 times) OK OK OK Bending 0.1 mm 0.2 mm Unmeasurable0.3 mm Unmeasurable stability Recovery OK OK NG OK NG

Referring to Tables 1 to 4, the films prepared by using the coatingcomposition of the present invention exhibited excellent characteristicsin terms of each physical property, and showed excellent durability andstability in bending tests including high hardness. On the other hand,the films of Comparative Examples had lowered pencil hardness or did notexhibit sufficient bending durability and stability to be suitable forflexible films.

What is claimed is:
 1. A coating composition comprising: a 3- to6-functional acrylate-based binder; a 7- to 20-functional urethaneacrylate-based binder; a photoinitiator; and inorganic fine particleshaving a bimodal particle: size distribution including a first inorganicfine particle group having d₅₀ of 20 to 35 nm and a second inorganicfine particle group having d₅₀ of 40 to 130 nm.
 2. The coatingcomposition according to claim 1, wherein the 7- to 20-functionalurethane acrylate-based binder has an acrylate equivalent weight of 200to 1,500 g/mol.
 3. The coating composition according to claim 1, whereinthe weight ratio between the 3- to 6-functional acrylate-based binderand the 7- to 20-functional urethaneacrylate-based hinder is 1:9 to 4:6.4. The coating composition according to claim 1, wherein the coatingcomposition includes the 3- to 6-functional acrylate-based binder in anamount of 10 to 50 parts by weight, the 7- to 20-functional urethaneacrylate-based binder in an amount of 40 to 70 parts by weight, thefirst inorganic fine particle group in an amount of 5 to 50 parts byweight, the second inorganic fine particle group in an amount of 5 to 50parts by weight and the photoinitiator in an amount of 0.1 to 5 parts byweight, based on 100 parts by weight of the coating composition.
 5. Thecoating composition according to claim 1, wherein the first inorganicfine particle group and the second inorganic fine particle group are thesame or different and each independently surface-modified with any oneor more silane coupling agents selected from the group consisting of(meth)acrylsilane, iacroxysilane, vinylsrlane, epoxysilane, andmercaptosilane.
 6. The coating composition according to claim 1, whereinthe first inorganic fine particle group has d₅₀ of 10 to 19 nm and d₉₀of 25 to 45 nm, and the second inorganic fine particle group has d₅₀ of2.5 to 110 nm and d₉₀ of 60 to 150 nm.
 7. The coating compositionaccording to claim 1, wherein the weight ratio between the firstinorganic fine particle group and the second inorganic fine particlegroup is 9:1 to 3:7.
 8. The coating composition according to claim 1,wherein the coating composition is used for preparation of a flexibleplastic film.